This invention relates to an apparatus for measuring the velocity of an object through the medium of an ultrasonic wave, and more particularly to an apparatus for measuring, in a real-time manner, the velocity of a blood stream in an organism.
Various types of apparatus for detecting the velocity of an object on the basis of the Doppler effect have been known. In an apparatus operated in accordance with a pulse Doppler method (such as that reported in "IEEE TRANSACTION ON SONICS AND ULTRASONICS" Vol. SU-17, No. 3, July 1970, p.p. 170 to 185"), ultrasonic pulses are repeatedly transmitted toward an object to be measured, and a signal obtained from a received wave undergoes a time gate process, thereby ascertaining a portion of the object measured.
In this pulse Doppler method, if the time interval at which the transmission of the ultrasonic pulse is repeated is T, the measurable maximum doppler shift frequency fd is 1/2 T. On the other hand, if the propagation velocity of an acoustic wave transmitted toward the object is S, the depth D upto which the object can be measured (hereinafter referred to as the measurable depth) is S/2. Accordingly, the product of the fd and D is S/4 which is constant. The measurable velocity or measurable depth is thus limited.